The invention relates to the field of RF power detectors and controllers, and in particular to RF power detectors and controllers used in mobile handset terminals for wireless applications.
The rollout of the new 2.5 and 3G wireless systems presents unique challenges to mobile handset designers. In order to reap the full benefit of expanded capacity and data bandwidth, the new handsets must work on both the new systems as well as the old. One of these new systems is the so-called Enhanced Data Rates for Global Evolution (EDGE). The EDGE standard is an extension of the Global System for Mobile Communications (GSM) standard.
EDGE increases the data rate over that available with GSM by sending more bits per RF burst. More bits are sent in EDGE by using a modulation scheme based on 8-phase shift keying (8-PSK), which provides an increase over GSM's Gaussian minimum shift keying (GMSK) modulation format. In the EDGE modulation scheme, the 8-PSK constellation is rotated 3π/8 radians every symbol period to avoid the problems associated with zero crossings. In contrast to GMSK's constant amplitude envelope, the added rotation factor in the EDGE modulation scheme results in a non-constant amplitude envelope. This non-constant amplitude envelope presents some difficulties with regard to RF power control. These problems are exacerbated by the desire to have one transmitter that can be used for both the GSM and EDGE standards.
FIG. 1 shows a prior art power detection and control system 100 for GSM systems. GSM constant envelope signals are input to and amplified by a power amplifier (PA) 102. Signals output by power amplifier 102 are coupled by a directional coupler 108 into some form of demodulating detector 104, typically a logarithmic amplifier (log amp), which translates the power of the output signals into a voltage. FIG. 2 shows the output 200 of demodulating log amp 104 when a GSM signal 202 is applied at the input. As shown in FIG. 2, the voltage 200 output by log amp 104 is a DC voltage because GSM signals have a constant amplitude envelope. The DC output of log amp 104 is then compared to a ramp control signal using a high gain, frequency limited differential amplifier 106. Based upon the comparison, differential amplifier 106 outputs an error signal, which is filtered by a filter capacitor 107 (amplifier 102 is configured as an integrator) to create a PA ramp voltage that is used to control the power output of PA 102. This creates a closed loop system that will set the output power to a level defined by the ramp control signal. The system is defined by the power control slope in dB/V and the 0V intercept point in dBm. The frequency response of this closed loop system must be fast enough to provide an adequate rise time, and slow enough to avoid ringing or instability.
Some unique problems arise when an EDGE signal having a non-constant amplitude envelope is applied to the GSM control loop 100. The EDGE system standard requires that PA 102 ramp up and down with the same speed as for GSM (28uS). Thus, the loop must have a response faster than 35 kHz. However, unlike the GSM signal, the EDGE signal contains an AM component resulting from the non-constant amplitude envelope. Logarithmic amplifier 104 will detect this modulation and vary the output DC voltage accordingly. This is illustrated in FIG. 3, which shows the output 300 of demodulating log amp 104 when an EDGE signal 302 is applied at the input. As shown in FIG. 3, the voltage 300 output by demodulating log amp 104 has a voltage ripple component that results from the non-constant amplitude envelope, in addition to the constant DC voltage component. The resulting voltage ripple results in as much as a 20 dB variation in amplitude. Because the DC voltage is varied according to the AM component, the negative feedback of the closed loop system will work to eliminate the AM information in the EDGE signal.
Several methods for solving this problem have been proposed. First, two different filters could be used. A fast filter is used for the power ramping section of the signal, and a second, slower filter is switched in when the AM modulation begins. This solution will still allow some AM variation to leak into the loop since the filter will require some amount of time to create an average and settle. Another approach is to remove the second filter and simply hold the DC To voltage constant during the AM modulation phase. This track and hold function will not have the same problem with settling time and no AM ripple will leak into the loop. However, the system will be running open-loop while the hold function is engaged. Thus, no corrections can be made to the PA output power during the RF burst to compensate for external influences (battery fluctuations, temperature, etc.). The finite nature of the data is another problem. Only 102 symbols of data are transmitted per frame. This small sample size means that there will be variations in the number and type of phase transitions per frame thus changing the peak to average ratio of the signal and the resulting average power per frame. An open loop system will not be able to adjust for these variations.